This invention concerns an ignition device, particularly an ignition coil with an internal diagnostic lead orthogonal to the secondary conductor, that acts as an antenna, onto which a signal is induced that possesses the exact frequency and relative time-domain characteristics as the secondary voltage.
The most common form of ignition system employed in engine-driven field gathering applications is a capacitive discharge system, with one ignition coil and spark plug per cylinder. In these applications, the engine typically runs continuously and unattended. It is therefore important that the ignition system require as little maintenance as possible to minimize down time. The ignition system components requiring the most service or replacement are on the secondary side of the ignition coil: spark plugs, plug wires, ignitions coils, and insulating boots.
Known methods exist for monitoring the secondary side of an ignition coil. U.S. Pat. No. 5,623,209, entitled xe2x80x9cDiagnostic System for Capacitive Discharge Ignition Systemxe2x80x9d describes such a system. Unfortunately, this system only infers information about the secondary side of the ignition coil by monitoring the voltage across the storage capacitor, it does not directly monitor the secondary side. Because of this, the types of conditions this system can detect on the secondary side are limited.
A traditional method for directly monitoring the secondary side of the ignition utilizes a test lead that can be clipped around the spark plug wire. But this method is impractical in the field, as external equipment is required to gather diagnostic information. Further, at times, there may be no location onto which the test lead can be attached
U.S. Pat. No. 5,461,315, entitled xe2x80x9cHigh Voltage Transformer for the Continuous Monitoring of High Voltage Characteristics in a Combustion Engine,xe2x80x9d describes a system that overcomes some of the problems associated with the traditional test lead by integrating an isolated conductor within the ignition coil near the secondary. This isolated conductor forms a capacitor with the secondary high voltage conductor. Using an external capacitor, a capacitive voltage divider is formed to measure variations in voltage dependent on the high voltage of the secondary. Because this method relies on the amplitude of the voltage applied to the conductor surface relative to actual secondary voltage, this method requires precise component location and complicated interface scaling circuitry. Because this method is implemented by constructing a capacitor, it is also affected by circuit loading.
Finally, U.S. Pat. No. 5,156,127, entitled xe2x80x9cMethod for Optimizing Plug Firing Time and Providing Diagnostic Capability in an Automotive Ignition System,xe2x80x9d describes a method for determining the firing time of a spark plug. This method, however, requires an external circuit to sense the rise in current on the secondary side, while the present invention integrates an isolated lead in the ignition coil.
It is therefore an object of the present invention to provide a simple and inexpensive ignition coil with a diagnostic lead that provides a signal with the exact frequency and relative time domain characteristics as the secondary voltage.
It is a further object of this invention to provide interface circuitry that accepts the diagnostic signal as an input and provides a signal that is an indication of the precise moment of firing. This signal will be used in an ignition system to terminate a clocking signal that was started at the initiation of the firing pulse.
The precise time of the firing point is then used by the system for determining several key performance factors such as the spark plug demand voltage, the success or failure of the ignition event, and the amount of excess energy used. It is therefore a further object of the present invention to control the energy delivered to the spark plugs, thus extending their life.
The present invention deviates from previous methods that rely on predictable scaling of signal amplitude to the actual secondary amplitude by concentrating on the timing of the diagnostic signal. The amplitude of the signals generated in these other methods is used for diagnostic purposes and for continuous monitoring. In the present invention, the timing of the firing point indication is the only characteristic desired from the diagnostic signal.
It is a still further object of the present invention that the system can distinguish between the compression and exhaust strokes of a cylinder in four cycle engines without the need for an external cam reference device.
Finally, because the diagnostic leads are high impedance antennas, rather than capacitive elements, it is an object of the present invention to provide ignition coils with diagnostic leads that can be connected to form a common node, without interfering with normal engine operation. This reduces overall system cost by requiring only one signal interface circuit.